X-ray fluorescence (XRF) instruments measure properties of material by irradiating the material with x-rays or gamma rays and analyzing the fluorescent radiation to determine specified properties. The term “x-ray,” as used herein and in any appended claims, refers broadly to penetrating radiation that is generated either by radioactive sources, or by instruments such as x-ray tubes, and encompasses within the term all forms of penetrating radiation including gamma rays. The specified properties to be determined may include the elemental composition of the irradiated object, or the distribution of a particular element in the near surface of the object, or the density of the object, or the morphology of a particular layer of material.
XRF instrumentation is the subject of U.S. Pat. Nos. 5,274,688, 5,390,229, 5,396,529, and 5,461,654, all to Grodzins, and all incorporated herein by reference. In those patents, Grodzins showed how it was possible to measure the concentration of an element in a single layer, with particular application to lead that is a constituent of a layer of lead-based paint.
As used herein and in any appended claim, the term “areal density” of an object, or of material forming a portion of an object, is defined as the thickness of the material multiplied by its density. For example, a layer of copper (density of 9 gm/cm3), that is 10−4 cm thick, has an areal density of 900 μg/cm. Conversely, a determination that the thickness a copper layer with an areal density of 900 μg/cm2 allows an inference that the thickness must be 10−4 cm (i.e., 1 μm).
In the prior art, where thicknesses of multilayers are to be measured using x-ray fluorescence, a beam of x-rays impinges on a multilayer sample and produces an energy versus intensity spectrum of fluorescent radiation. The energy (or, equivalently, the wavelength) of the sharp peaks in the XRF spectrum corresponds to the unique characteristic x-ray energies of the elements in the sample, while the intensity of the x-rays in the sharp peaks gives a measure of the areal density of the elements in the layers. The derived areal density, together with a known density of known material, may be used to yield the thickness of the layer, but only if the composition and ordering of overlying layers is known. (In cases where the sample is not layered but homogeneous, the intensity of the characteristic x-rays of an element gives a measure of the concentration of the element; homogeneous samples, however, are not of concern in the present invention.)
It would be valuable to establish a capability to measure the thicknesses of elements in layers that are part of a multiple layer stack of materials, in particular, layers that are buried beneath other layers, and, additionally, to be able to measure the absorption of the burying layers, as part of the same measurement.